1. Field of the Invention
The present invention relates to a process for the mixed production of high or low purity oxygen using a double reboiler and double column arrangement.
2. Description of the Background
The double reboiler and double column process was developed to produce relatively low purity oxygen at purities below 97% oxygen content. This process is described in U.S. Pat. No. 3,113,854 and in the Electric Power Research Institute (ERPI) Study 3499. Additionally, U.S. Pat. No. 4,895,583 discloses a modification of an earlier process in order to improve the overall recovery of oxygen.
Another example of a conventional double column and double reboiler process for producing relatively low purity oxygen is that disclosed in U.S. Pat. No. 4,705,548. However, this process is disadvantageous as it requires the use of a compressor for nitrogen recycling.
Attempts to produce high oxygen purities, such as above 97% oxygen content, using the double reboiler and double column methodology have not been considered interesting due to the sharp reduction in process efficiency and loss of recovery which is entailed. These drawbacks occur because it is necessary to perform the very difficult argon/oxygen separation by cryogenic distillation for oxygen contents of higher than 97%.
At present, in order to obtain an oxygen content of greater than 97%, the single reboiler, double column process is usually used. Unfortunately, this process has a much higher specific power consumption than the double reboiler and double column process.
It would be extremely advantageous if a process were known for producing oxygen at both low and high purity. Frequently, an on-site oxygen facility may produce a gaseous oxygen product at 95% purity for a pipeline customer, yet local demand may require a liquid oxygen product having 99.6% oxygen content which is a normal standard grade for liquid oxygen.
Furthermore, in situations where significant liquid production is required a traditional single reboiler process is used whereby air, expanded through an expander, is injected into the high pressure column. This affords a good recovery of oxygen since the majority of the air is still being treated in the high pressure column and does not by-pass the first step of distillation.
However, this feature has a major drawback because of the relatively high discharge pressure of the expander which is the high pressure column pressure. The feed air pressure must, therefore, be maintained at a high pressure to achieve the required refrigeration. In such situations, high power consumption would be needed to produce the required liquid.
Thus, a need exists for a process for producing oxygen at both low and high purity, more precisely, low purity gaseous oxygen and high purity liquid oxygen, and which is not characterized by a prohibitively high specific power consumption.